Public Release: 12-Mar-2013
AGU journal highlights - March 12, 2013

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Water Resources Research (WRR), and Journal of Geophysical Research-Biogeosciences, (JGR-G).

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://onlinelibrary.wiley.com/ and inserting into the search engine the full doi (digital object identifier), e.g. 10.1002/grl.50214. The doi is found at the end of each Highlight below.

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1. Canadian Arctic glacier melt accelerating, irreversible

Ongoing glacier loss in the Canadian high Arctic is accelerating and probably irreversible, new model projections by Lenaerts et al. suggest. The Canadian high Arctic is home to the largest clustering of glacier ice outside of Greenland and Antarctica--146,000 square kilometers (about 60,000 square miles) of glacier ice spread across 36,000 islands. In the past few years, the mass of the glaciers in the Canadian Arctic archipelago has begun to plummet. Observations from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites suggest that from 2004 to 2011 the region's glaciers shed approximately 580 gigatons of ice. Aside from glacier calving, which plays only a small role in Canadian glacier mass loss, the drop is due largely to a shift in the surface-mass balance, with warming-induced meltwater runoff outpacing the accumulation of new snowfall.

Using a coupled atmosphere-snow climate model, the authors reproduced the observed changes in glacier mass and sought to forecast projected changes given a future of continued warming. Driving the model with a climate reanalysis dataset for the period 1960 to 2011 and with a potential future warming pathway, the authors find that their model accurately reproduces observed glacier mass losses, including a recent up-tick in the rate of the ice's decline.

The authors calculate that by 2100, when the Arctic archipelago is 6.5 Kelvin (14 degrees Fahrenheit) warmer, the rate of glacier mass loss will be roughly 144 gigatons per year, up from the present rate of 92 gigatons per year. In total, the researchers expect Canadian Arctic archipelago glaciers to lose around 18 percent of their mass by the end of the century. Given current warming trends, they suggest that the ongoing glacier loss is effectively irreversible.

In about a third of the global ocean, the abundance of life is limited by a dearth of biologically available iron. The supply of iron to a region that is depleted in this important nutrient can stimulate algal productivity, and can result in a temporary boom in biological activity. For much of the surface ocean, the wind-borne transport of iron-rich dust and the upwelling of nutrient-filled water are the major sources of iron. Another potentially important source is the deposition of the iron-rich ash produced by volcanic eruptions. Though satellite observations and modeling work suggest that volcanic ash could seed life in such a way, there have been only a limited number of direct observations of the effects of ash deposition on surface ocean waters.

Thanks to a bit of serendipitous scheduling, Achterberg et al. conducted a series of research cruises in the Iceland Basin region of the North Atlantic Ocean both during and after the month-long eruption of Iceland's Eyjafjallajökull volcano in the spring of 2010. Three cruises allowed the authors to undertake measurements of surface ocean iron concentration before, during, and after the eruption in a region directly affected by the towering ash plume. Beneath the plume, the authors found peak dissolved iron concentrations up to 10.2 nanomolar, compared to 0.23 to 0.45 nanomolar detected before ash deposition. Using a model of the ash plume trajectory and ash deposition rates, along with measurements of iron dissolution, the authors calculated that up to 570,000 square kilometers (220,000 square miles) of North Atlantic waters could have been seeded with at least 0.2 nanomolar of iron. In controlled biological incubation experiments, the authors added volcanic ash collected under the plume to sea water, and find that iron leached from the ash could drive an increase in biological productivity and a draw-down of nutrient levels.

Most people think of seismometers as ground-based instruments, but earthquakes can be detected by satellites too, as demonstrated by Garcia et al. using data from the European Space Agency's Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission. They call GOCE "the first seismometer in orbit around the Earth," because it was able to detect infrasonic waves in the atmosphere generated by the 2011 Tohoku earthquake.

The shaking of the ground during an earthquake produces acoustic waves that propagate vertically in the atmosphere. By using the precise vertical acceleration measurements of the GOCE satellite, which orbits Earth in the thermosphere at about 270 kilometers (about 170 miles) altitude, as well as deducing air density variations encountered by the satellite, the scientists can detect these waves. The earthquake-generated atmospheric waves can be distinguished from other types of gravity waves in the atmosphere because the ratio of the satellite's vertical acceleration to the perturbation in air density is higher for these post-seismic waves than for usual gravity waves in the atmosphere.

The researchers also modeled these atmospheric waves generated by the Tohoku earthquake; compared the amplitude, timing, and waveshape of the modeled waves to that deduced from the GOCE data; and find the model and data agree well. Wave travel time delays relative to synthetic data were ascribed to lateral variations of both seismic velocities in the solid Earth and sound speed in the atmosphere. The study highlights the potential for future satellite-based observations of earthquakes.

Water resources can become strained by both natural factors such as drought and by human factors such as unsustainable use. Water resource managers can develop practices to reduce overuse of water resources, but they cannot prevent droughts, so distinguishing the causes of water stress can be useful. However, since the two factors often occur at the same time, separating them can be difficult.

Van Loon and Van Lanen propose an observation-modeling framework for distinguishing natural and human effects on the hydrological system. They define "drought" as a temporary period of lower than normal water levels or streamflow caused by natural weather variability, and "water scarcity" as the unsustainable overexploitation of water resources by humans, in which water demand is higher than availability.

Using models, they simulated the natural system without human influence, and then compared the results with observations to separate out the human-induced effects on water resources. They identify anomalies (deviations below some threshold) in both the simulated natural system with no human influence and the observational data, in which both drought and water scarcity manifest themselves. They demonstrate the framework in a test case in the Upper-Guadiana catchment in Spain, and show that in this region for the period from 1980 to 2000, the effects of human intervention on groundwater supplies were four times larger than the effects of natural drought. The framework could be applied to other regions to help water resource managers make better decisions for sustainable water use.

Geologic carbon sequestration, in which carbon is captured and stored underground, has been proposed as one way to mitigate the climatic effects of carbon dioxide emissions. One method of geologic carbon sequestration is to inject carbon dioxide in aqueous solution into rocks. However, as the solution fills the pore space in the rocks, the fluid pressure on the rocks increases, potentially increasing the risk of earthquakes. Another option would be to inject carbon dioxide solutions into mafic rocks; the silicate minerals in these rocks react with the carbon dioxide, leaving solid carbonate reaction products, which decrease the amount of pore fluid.

To determine how mineral carbonation reactions affect seismic risk, Yarushina and Bercovici created a simple model to see how these reactions influence stress on the rock during and after carbon dioxide injection. Their model shows that the chemical reactions reduce fluid pore pressure and distribute stress on the minerals over a larger area. They conclude that mineral carbonation in mafic rock could minimize the seismic risk of carbon sequestration by underground injection as long as fluid pumping rates do not exceed a critical value.

Plants need nitrogen to grow, and nitrate is a common fertilizer ingredient, but high levels of nitrate contamination in drinking water sources can cause health problems. It is generally known that nitrogen flows through watersheds from upslope areas down to streams, but the relationships between upslope soil solution or groundwater nitrate concentrations and stream water nitrate levels--and the ways in which land use changes may alter this relationship--are not fully understood.

Sudduth et al. analyzed published studies of 62 watersheds to see if a consistent relationship between upslope soil solution or groundwater nitrate concentrations and stream water nitrate concentrations exists and whether ecosystem disturbances such as fire or land use changes such as clearing forests for agriculture or urbanization affected the relationship between soil and stream water nitrate concentrations.

For 40 undisturbed forest watersheds and 10 disturbed forest watersheds, they find that stream water nitrate concentrations are typically about half that of soil solution nitrate concentrations, indicating that a significant amount of nitrate is removed as water passes through watersheds to streams. For the 12 watersheds in their study that had significant agricultural or urban development, there is less reduction in nitrate concentrations between soil solutions and stream waters, suggesting that in human-dominated landscapes, upland soils and riparian zones are less efficient at removing nitrate than in undisturbed ecosystems.

The study suggests that, in general, stream water nitrate concentrations can provide an indication of upland soil solution or groundwater nitrate levels. The authors conclude that undisturbed watersheds have a significant capacity to remove nitrate, but land use changes tend to diminish the efficiency of nitrate removal from watersheds or alter the flow paths of nitrate.

7. Devastating East African drought made more likely by climate change

In 2011 a powerful drought gripped East Africa. The failure of both the 2010 fall rains and the 2011 spring rains caused a drought that, stacked on an already unstable political climate, caused a famine that led to hundreds of thousands of deaths. Whenever an extreme weather event strikes a population--a drought, a hurricane, or a powerful flood--questions arise as to whether ongoing global climate change is complicit. Most scientists, when confronted with such questions, suggest that no one act of weather can be attributed to the long-term statistical shifts that make up a change in climate. The nascent field of event attribution, however, is seeking to provide a more satisfying answer to this question. Using climate modeling techniques, researchers estimate how the probability or magnitude of a specific extreme event--in this case, the failure of the East African rains--was affected by climate change.

Comparing modeled East African precipitation patterns in a world affected by anthropogenic forcings against a climate change-free scenario, Lott et al. find that the probability of failure of the 2011 East African spring rains was increased by climate change, though the various models used disagreed on the exact size of the increase. The authors suggest that the reduction of the spring rains hinged on the rise in sea surface temperature caused by anthropogenic climate change. They find that the failure of the 2010 fall rains, however, was due not to anthropogenic climate change but to the ongoing La Niña conditions.

Tree ring records indicate that in 774-775 CE, atmospheric carbon-14 levels increased substantially. Researchers suggest that a solar proton event may have been the cause. In solar proton events, large numbers of high-energy protons are emitted from the Sun, along with other particles. If these particles reach Earth's atmosphere, they ionize the atmosphere and induce nuclear reactions that produce higher levels of carbon-14; the particles also cause chemical reactions that result in depletion of ozone in the ozone layer, allowing harmful ultraviolet radiation to reach the ground.

A previous group of researchers suggested that to cause the observed eighth century carbon-14 increase, a solar proton event would have had to be thousands of times larger than any that has been observed from the Sun. However, Thomas et al. believe that group's calculations were incorrect. They modeled the atmospheric and biologic effects of three solar proton events with different energy spectra and fluences (number of protons per area). They find that an event with about 7 or more times greater fluence (depending on the spectrum) than an observed October 1989 solar flare event could explain the 774-775 CE carbon-14 enhancement. With a hard (high-energy) spectrum, an event with this fluence would result in moderately damaging effects on life but would not cause a mass extinction. They rule out an event with a softer spectrum because such an event would cause severe ozone depletion and mass extinction, which were not observed in the eighth century. The authors estimate that solar proton events of this magnitude occur on average once in a thousand years, and more often if the estimate is based on astronomical observations of flares on Sun-like stars. They note that although that may seem low, such an event would have severely damaging effects on the technology on which society relies.

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